I think the gaps are where the page has been erased and restored. See the IBM History Flow page for more details and examples.

I think this has been mentioned in the press already, but I confirmed with Jimmy Wales that a study done by IBM (The group that did the history flow work) tried to measure how quickly vandalism on Wikipedia was identified and corrected. They searched for pages where suddenly all of the content disappeared or a huge amount was deleted. They found that the median time for such a page to be restored was 5 minutes. This did not take into the account the process that where Wikipedians often refactor or move pages and redirect them which would show a similar behavior. So the median time is probably less than 5 minutes. In the context of our discussion about Wikipedia authority, I think this is quite an interesting and impressive statistic.